The growing medium is what you are cultivating your marijuana in and it can be a substance such as Rockwool, coco coir, vermiculite, soil, water, etc. Soil mixes are used, or hydroponics, and sometimes soilless mixes. Often with soil mixes it will be a soil or compost soilless mix (such as perlite and coco coir), compost soil or hydroponics soil is the most popular and easiest to use for growing mediums; it is natural and available basically everywhere.
Soil that is good contains a natural amount of nutrients which will sustain your plant in its first few weeks of life. Soil mixes are an alternative to hydroponic growing which maintains and supports your roots in water for many stages of the plant’s life after the initial growth phases.
There are several different grow mediums. An aroma of complex-smelling buds will be the result of cultivating in organic composted living soil. There are also marijuana-friendly soil mixes such as the Fox Farms Ocean Forest soil mix which contains enough nutrients to last for the first month of your seedling's life whereas when the plant gets older it is a recommended practice to place nutrients in the hydroponic water.
A soilless mix usually is made primarily of perlite and coco coir but can technically include vermiculite, though not as commonly. All mixes that do not have soil are considered hydroponic growing as there is no medium. But a soilless compound is often thought of as a mixture between soil and hydro as you will receive the best part of both worlds. A coco and perlite mix is recommended for your first attempt at growing.